1. Field of the Invention
The present invention relates generally to an ink jet recording apparatus arranged to effect recording by discharging a recording liquid such as ink and, more particularly, to an ink jet recording apparatus provided with means for preventing the failure of ink discharge from occurring due to dust particles clogging in discharge openings (or orifices), excessively condensed ink, bubbles which might be contained in the ink, or the like.
2. Related Background Art
FIG. 1 is a schematic perspective view showing one example of the recording head used in such an ink jet recording apparatus. The illustrated recording head comprises a discharging element 1 including a multiplicity of liquid channels which are arranged side by side at an extremely narrow pitch. As will be explained in detail below, each of the liquid channels includes an energy generator such as a heat-generating device for generating the energy required to discharge a recording liquid (or ink). The discharging element 1 also includes orifices 10 which are formed in the upstream end portions of the respective liquid channels, a common liquid chamber (to be described later) for holding ink to be supplied to the respective liquid channels, and so on. The ink is discharged from each of the orifices 10 to form a recording droplet.
The illustrated recording head further comprises a base plate 3 to which the discharging element 1 is fixed, as by an adhesive, and a front plate 2 fixed to one end face of each of the discharging element 1 and the base plate 3 by means of fastening members such as bolts (not shown). The front plate 2 has an aperture through which the orifices 10 can directly oppose a recording medium (not shown). The recording head also includes elbow-shaped connecting members 15 through which ink is introduced into the common liquid chamber defined in the discharging element 1, filter units 17 disposed midway along individual ink supply paths extending from an ink supply source such as an ink tank, which will be described in connection with FIG. 2, and supply pipes 16 which connect the connecting members 15 with the filter units 17, respectively. These members 15, 16 and 17 form a part of an ink supply system which will be described later.
FIG. 2 is a schematic block diagram showing the discharge failure recovery system used in a typical ink jet recording apparatus. During a normal recording mode, a cap member 4 is held in an appropriate position which does not hinder the recording operation. A valve 31 is kept open, while valves B2 and B3 are kept closed. In this state, ink is supplied from an ink tank 6 to the discharging element 1 through the valve B1 owing to a so-called capillary phenomenon.
When a discharge failure recovery process must be executed, the cap member 4 is moved into contact with the discharging element 1 and the valve B1 is closed, while the valves B2 and B3 are opened. In this state, a pump 7 is activated to feed ink from the ink tank 6 into the ink supply paths by pressure, thereby supplying the pressurized ink to the discharging element 1 and forcing the ink to jet through the orifices 10. Dust particles, excessively condensed ink, bubbles or similar foreign matter which may cause the discharge failure are also expelled from the discharging element 1 together with the jets of ink. For example, as shown in FIG. 4 which will be discussed later, if miniature bubbles a enter some liquid channels 12, they will be expelled from the discharging element 1 through the orifices 10 together with ink jets by the operation of the pump 7. The ink which has jetted from the orifices 10 is received by the cap member 4 and introduced into a waste ink tank 5.
FIGS. 3 and 4 are a vertical sectional view and a horizontal sectional view, respectively, of the recording head of FIG. 1. FIGS. 3 and 4 illustrate the state of the cap member 4 being maintained in contact with the front plate 2 to cover the entire aperture in which the orifices 10 are located.
The liquid channels 12 extend from the respective orifices 10 to an eaves-like end portion 13 which faces the common liquid chamber 14.
The term "ink path" which is used hereinafter is defined as embracing a plurality of liquid channels 12 and the common liquid chamber 14 with which the liquid channels 12 communicate in common.
Each of the liquid channels 12 includes an energy-generating device 11 for generating the energy required to discharge ink, and the energy-generating device 11 utilizes a heat-generating device. (In FIG. 4, only one energy-generating device 11, which is provided in the liquid channel 12 located at one end, is shown for the purpose of illustration.) A filter 100 is disposed in the filter unit 17 in order to eliminate miniature dust particles, bubbles or the like.
However, since the above-described arrangement is designed to effect recovery from a discharge failure by expelling ink through discharge openings (or orifices), no satisfactory result may be obtained in the case of, for example, tapered liquid channels such as those shown in FIG. 4. For instance, it will be impossible to remove dust particles which are larger than the discharging openings.
FIG. 5 is a block diagram showing the fluidic equivalent circuit of a recovery arrangement according to the background art when a discharge failure recovery process is being executed. As can be seen from the figure, during the discharge failure recovery process, the following relationship is established: EQU .DELTA.p=q.times.R1+n.times.q(RH+RC+RF+RS) EQU .thrfore.q=.DELTA.p/{r1+n(RH+RC+RF+RS)}
where .DELTA.p=pressure, n=number of liquid channels 12, R1=flow resistance per liquid channel 12, RH=flow resistance of the eaves-like end portion 13, RC=flow resistance of the common liquid chamber 14, RF=flow resistance of the filter units 17, RS=liquid resistance of the portion, excluding the filter units 17, between the ink tank 6 and the common liquid chamber 14, and q=flow rate in each liquid channel 12 when the pressure .DELTA.p is applied.
It is common practice to design the ink supply system so that R1&gt;RH+RC+RF+RS can be satisfied. However, if this relationship is applied to, for example, a so-called full multiple type (full line type) of recording head, namely, a recording head of the type having a plurality of liquid channels which are arrayed over a range corresponding to a recording width, the number n of liquid channels 12 increases and the flow rate q per liquid channel 12 decreases to an extremely small extent. If foreign matter such as dust particles or the bubbles a as shown in FIG. 4 enter a particular liquid channel 12, the flow resistance thereof will increase. As a result, the flow rate across the liquid channel in which the discharge failure has occurred is substantially reduced compared to the flow rate across a normal liquid channel. For these reasons, even if any discharge failure is to be eliminated with the discharge failure recovery system according to the background art, it is occasionally impossible to restore a liquid channel which has suffered the discharge failure to a normal state with a recovery operation. There is also a case where the recovery operation must be repeated many times until the discharge failure is recovered.
In addition, to overcome such flow resistance, it is necessary to increase pressure to be applied to the liquid channels and a high-pressure pump must therefore be prepared as the pump 7. As a result, the total amount of ink consumed may increase and it is also required that the strength of the joint portion of each member be increased to a level which can withstand such large pressure.